core: drop two manifest fields that were never wired up

`SystemdSpec.readiness` (ReadinessHttpGet) and `SystemdSpec.requires` were
parsed and validated but read nowhere — no generator, CLI or API touched
either. Both looked load-bearing and were not:

- readiness duplicated `expose.http.health_path`, which wildpc-api already
  polls. wild-life-api.yaml declared both spellings of the same /health
  probe; only one did anything.
- requires would mean systemd `Requires=`, a lifecycle binding (stop
  postgres, stop every dependent). `manage.systemd.after` already gives the
  ordering callers actually want, and a deployment's own `requires:` is the
  relationship model, not systemd wiring.

Unknown keys parse as ignored, so existing deployment YAML keeps loading.

cli/src/wildpc_cli/manifest.py

@@ -18,7 +18,6 @@ from wildpc_core.manifest import (  # noqa: F401 — explicit re-exports for typ
     PathDeployment,
     ProgramSpec,
     Reach,
-    ReadinessHttpGet,
     RemoteDeployment,
     Requirement,
     RestartPolicy,